1. Field of the Invention
The present invention is generally related to matrix-type liquid crystal light valve (LCLV) displays and their associated drive circuitry and, particularly, to the driver circuit utilized for activating each of the pixels of a matrix LCLV display.
2. Description of the Prior Art
In the development of high pixel density matrix LCLV displays, there is a growing need for a pixel driver circuit having an optimal circuit design. The needs that must be met include low power dissipation, minimum circuit component count, minimum size components, minimum electrical contacts to and within the circuit, optional routing of electrical leads and placement of the contacts to minimize crossovers, flexibility to provide either DC or true AC activation of the liquid crystal material, and the capability to ensure time and temperature stable operation of the liquid crystal material.
Prior existing pixel driver circuits can be characterized by the number of transistors present in the designs. Single transistor driver circuits typically include a strobe transistor for selectively passing data from a data line to a storage capacitor. The discharge path of the storage capacitor passes through the liquid crystal pixel, thereby providing contrast control dependent on the voltage potential established across the storage capacitor. Due to the necessarily short strobe period associated with high density matrix LCLV displays, the data current through the strobe transistor (high multiplexing ratio) may be extremely high. Also, the exponential rate of discharge of the data storage capacitor is highly dependent on time due to the low, variable pixel impedance, thus resulting in a relatively unstable pixel contrast characteristic. Naturally, there is also a time and temperature critical refresh period, based on the display frame rate, associated with such a driver circuit. Finally, true AC activation of the liquid crystal pixel cannot be accomplished utilizing this single transistor driver circuit, the pixel activation being inherently dependent on the polarity of the data potential.
A two-transistor driver circuit exists that overcomes a number of the limitations inherent in the design of the single transistor driver circuit. A strobe transistor is utilized to pass data from a data line to a data storage capacitor, as in the single transistor driver circuit. The discharge path of the data storage transistor, however, is into the gate electrode of a second transistor. The drain of the second transistor is connected to a power supply and the source of the transistor is connected to an electrode of the liquid crystal pixel so as to provide an alternate, data dependent current path through the liquid crystal pixel to ground. The placement of the gate electrode of the second transistor in the current discharge path of the data storage capacitor greatly reduces the needed charge storage capacity of the capacitor and the current handling capability of the strobe transistor. Thus, the size of the capacitor and the power dissipation of the circuit as a whole are reduced. However, all other limitations of the single transistor driver circuit remain essentially uncorrected in this two-transistor driver circuit.